Ultrasonic flow meters are often used in process and automation technology. They allow determination of the volumetric flow rate and/or mass flow in a pipeline in a simple way. Known ultrasonic flow meters often work according to the runtime difference principle. In the runtime difference principle, the different runtimes of ultrasonic waves, in particular ultrasonic pulses, so-called bursts, are evaluated relative to the direction of flow of the liquid. To this end, ultrasonic pulses are transmitted at a certain angle to the pipe axis, both in and against the direction of flow. Using the runtime difference, the flow rate and thus the volumetric flow rate can be determined if the diameter of the pipeline section is known.
The ultrasonic waves are generated or received by means of so-called ultrasonic transducers. For this purpose, ultrasonic transducers are firmly attached to the pipe wall of the relevant pipeline section. Clamp-on ultrasonic flow measurement systems are also available. In these systems, the ultrasonic transducers are pressed from outside of the measuring tube against the tube wall. A big advantage of clamp-on ultrasonic flow measurement systems is that they do not touch the measurement medium and can be mounted on an existing pipeline.
The ultrasonic transducers usually consist of an electromechanical transducer element, e.g. a piezoelectric element, and a coupling layer. In clamp-on systems, the ultrasonic waves are generated as acoustic signals in the electromechanical transducer element, and passed over the coupling layer to the pipe wall and then into the liquid. In inline systems, the acoustic signals are coupled to the measurement medium via the coupling layer.
Both in clamp-on systems as well as inline systems, the ultrasonic transducers are usually arranged in a common plane on the measuring tube, either on opposite sides of the measuring tube, in which case the acoustic signal traverses the measuring tube once along a secant, projected onto a tube cross-section, or on the same side of the measuring tube, in which case the acoustic signal is reflected at the opposite side of the measuring tube, whereby the acoustic signal traverses the measuring tube twice along the secant projected onto the cross-section through the measuring tube.
Two processes take place in the runtime difference principle. In the first process, a first ultrasonic transducer transmits acoustic signals that propagate through the medium to the measuring tube. The acoustic signals are received by a second ultrasonic transducer. In the second process, the second ultrasonic transducer transmits acoustic signals that are received by the second ultrasonic transducer. If a medium flows through the measuring tube, the first and the second processes result in different runtimes. The flow rate of the medium in the measuring tube is determined from these two runtimes.
Besides the acoustic signal, the transmitting ultrasonic transducer also generates an acoustic noise that is transmitted from an ultrasonic transducer housing to the measuring tube and to the receiving ultrasonic transducer via the measuring tube. Conventionally, the ultrasonic transducer housing with dampers made of polymers or elastomers is decoupled from the measuring tube to minimize the transmission of the acoustic noise. However, polymers or elastomers are of low strength and durability.